Sonic Tokens, also known as Sonic Coins, are gold coins with Sonic's face on them. They appeared in a few games, where they are either used to unlock features in or proceed with the levels. Actual Sonic tokens were used in SegaWorld amusement parks and arcades, before those places closed down.
Sonic Tokens first appeared in Sonic R. There are five tokens to collect in each course during Grand Prix. Collecting all of these tokens while coming in within the top three positions allows the player to race an unlockable character (except Robotnik and Super Sonic, who can be unlocked simply by claiming first place and collecting all Chaos Emeralds in all five tracks (Super Sonic for the latter, Robotnik for the former). Once the new character is beaten in this race, the character will be unlocked.
Collect five Sonic Tokens for these characters and beat them in a race to unlock them:
In Sonic Adventure, Sonic Tokens had the Sonic Team symbol on them (which still resembled Classic Sonic's face anyway). They are used in Sonic's version of Casinopolis, where he needs to collect four hundred Rings in order to finish the Action Stage. When Sonic makes the deposit using a switch (mechanical pincers grab and violently shake the rings from him) his collected rings will be converted into Sonic Tokens (1 Ring = 1 token) and pile up in the vault.
Knuckles can see Sonic's tokens in his version of Casinopolis as well.
During missions, some missions will require you to find Sonic Tokens which are described on the cards as medallions.
SegaWorld also used actual Sonic tokens. These were used to play the prize/ticket games in SegaWorld. Sonic tokens in London had Sonic's classic pose on one side, while the other side had the Sega logo.
The Sonic tokens in Sydney were more traditional to the games, as one side had Sonic's head while the other had SegaWorld Sydney's logo on it.
Various SegaWorlds also used Sonic Tokens to play the games.